This invention relates to a device for reproducing a halftone picture by scanning.
There are already known many methods for recording a halftone picture by means of a scanning recorder such as a color scanner for plate-making according to picture signals picked up by scanning an original picture. For example, Japanese Patent Lying-Open Publications Nos. 51-88301 and 51-150401 (Japanese Patent Applications Nos. 50-11386 and 50-73082) entitled "A machine and method for reproducing a halftone picture by scanning" have been filed by the same applicant assignee as the present invention.
In the former publication the width of the light beam perpendicular to the scanning direction is varied periodically by an aperture plate according to the picture signals, thereby recording a line of halftone dots in the direction of the scanning line.
In the latter publication No. 51-150401 the center of the recording light beam is periodically shifted in the axial direction of the recording cylinder according to the picture signals so as to record a line of halftone dots with a screen angle.
In the latter application, as illustrated in FIG. 1, the elements which are suitably arranged along the optical axis of the light beam 1, which is generated by a laser, include a first vertical deflection element 2, a cylindrical lens 3, a lenticular lens 4, a relay lens 5, an aperture plate 6 having an aperture 6a, a second vertical deflection element 7, a horizontal deflection element 8, a first focusing lens 9, a masking means 10 having an aperture 10a and a second focusing lens 11. A photo-sensitive material is scanned over the scanning area 12 by moving the photo-sensitive material while the above-mentioned three deflection elements 2, 7 and 8 diffract the scanning light beam according to the picture signals, thereby recording a halftone picture on the photo-sensitive material.
More specifically, the first and second vertical deflection elements 2 and 7 are positioned at conjugate points of the relay lens 5 and the deflection elements 2 and 7 are so controlled that their deflection angles are always the same, with the result that the light beam through the second deflection element 7 moves along the optical axis of the light beam 1 no matter how widely it is deflected between the two vertical deflection elements 2 and 7.
The light beam is flattened by the cylindrical lens 3 and the lenticular lens 4, and the flattened light beam passes through the aperture 6a of the aperture plate 6 adjacent to the relay lens 5. The width of the flattened light beam deflected by the vertical deflection elements 2 and 7 is varied according to the deflection angle. The aperture 6a is composed of a combination of three triangles; one of them constituting an upper portion thereof used to provide a beam of variable width as exemplified by the position indicated by arrow K.sub.2 in FIG. 2; the lower two triangles, and the masking triangle between them are used to provide a beam with an intercepted portion as exemplified by the position indicated by arrow K.sub.1 in FIG. 2. The upper and lower portions are thus complementary in function.
The center of the light beam passes through the second deflection element 7 and is shifted in the horizontal direction perpendicular to the optical axis by the horizontal deflection element 8 according to the screen angle desired. The light beam then passes through the first focusing lens 9, the aperture 10a of the masking means 10, and the second focusing lens 11, and is projected onto the photo-sensitive material.
This method is quite efficient, but it does have disadvantages. For example, the aperture 6a of the aperture plate 6 is rather complicated, the controlling of the deflection elements is also complicated, and a memory having a large storage capacity for the complicated control programs is required.